Leak-compensated pressure regulated volume control ventilation

ABSTRACT

This disclosure describes systems and methods for compensating for leakage when during delivery of gas to a patient from a medical ventilator in a pressure regulated volume control (PRVC) ventilation mode. The technology described herein includes systems and methods that compensate the delivery of PRVC ventilation for leakage in the patient circuit by using leak-compensated lung flows as well as respiratory mechanics (lung compliance and lung resistance) estimated in a manner that compensates for elastic and inelastic leaks from the ventilation system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/408,414, entitled “LEAK-COMPENSATED PRESSURE REGULATEDVOLUME CONTROL VENTILATION,” filed on Mar. 20, 2009, the entiredisclosure of which is hereby incorporated herein by reference.

INTRODUCTION

In mechanical ventilation, pressure regulated volume control (PRVC)ventilation is a type of pressure regulated treatment that provides thecapability to control the volume of gas delivered to the patient's lungsby adjusting the inspiratory target pressure. In PRVC, a selected volumeis delivered to a patient by changing the pressure of the respiratorygas. During PRVC ventilation, the ventilator will evaluate the volumedelivered to the patient and compare it against the desired volume setby the therapist. If delivered volume is less than the desired,therapist-selected volume, then the target pressure for the next breathwill be increased proportionally. On the other hand, if the deliveredvolume exceeded the desired volume, the target pressure will be lowered.

In one implementation of PRVC, the ventilator estimates the complianceof the patient's lungs and uses the estimated lung compliance tocalculate the target pressure that will result in the delivery of thepre-selected volume of gas. The magnitude and shape of the inspiratoryflow delivered by the ventilator will be a function of the patient lungcharacteristics, breathing pattern, and other ventilator settings.

The lung compliance and lung resistance of a patient may be collectivelyreferred to as the respiratory mechanics of the lung or, simply, thepatient's respiratory mechanics. Because PRVC relies on the patient'srespiratory mechanics when determining what pressure to provide for eachbreath, more accurate determination of respiratory mechanics isessential to performance of the ventilator when providing PRVCventilation.

Leak-Compensated Pressure Regulated Volume Control Ventilation

This disclosure describes systems and methods for compensating forleakage when during delivery of gas to a patient from a medicalventilator in a pressure regulated volume control (PRVC) ventilationmode. The technology described herein includes systems and methods thatcompensate the delivery of PRVC ventilation for leakage in the patientcircuit by using leak-compensated lung flows as well as respiratorymechanics (lung compliance and lung resistance) estimated in a mannerthat compensates for elastic and inelastic leaks from the ventilationsystem.

In part, this disclosure describes a method of compensating for leakagein a ventilation system during delivery of pressure regulated volumecontrol ventilation to a patient. The method starts with monitoring aninstantaneous flow of respiratory gas in the ventilation system based onone or more measurements of pressure and flow in ventilation system.Leakage of gas from the system is modeled as a first leakage componentthrough a first orifice of a fixed size and a second leakage componentthrough a second orifice of a varying size, in which the first andsecond leakage components are different functions of instantaneouspressure in the ventilation system. A leak-compensated delivered lungvolume is then estimated for at least one breath based on the one ormore measurements, the first leakage component and the second leakagecomponent. The leak-compensated delivered lung volume and apredetermined respiratory mechanics model are then used to estimate aleak-compensated lung compliance. A target pressure to be delivered tothe patient for a subsequent pressure-based breath is then calculatedbased on a desired lung volume, the leak-compensated delivered lungvolume and the leak-compensated lung compliance. The target pressure isthen delivered to the patient during the inspiratory phase of the nextbreath. The leak-compensated lung compliance may be estimated based onthe leak-compensated delivered lung volume and a pressure difference,such as the difference between an end inspiratory pressure of a firstbreath and an end expiratory pressure of the first breath.

The disclosure also describes a method of compensating for leakage in aventilation tubing system during delivery of gas from a medicalventilator to a patient. The method includes measuring leakage from theventilation tubing system during a first breath and calculating aleak-compensated delivered lung volume for the first breath based on theleakage. The method then estimates a lung compliance of the patientbased on the leak-compensated delivered lung volume and pressuremeasurements taken during the first breath. Ventilation is thendelivered to the patient in a second breath at a pressure determinedbased on a desired delivered lung volume, the leak-compensated deliveredlung volume and the leak-compensated lung compliance. As part ofmeasuring the leakage, the method may include identifying an inelasticleakage from the ventilation tubing system as a first function of atleast one of a pressure measurement and a flow measurement in theventilation system and identifying an elastic leakage from theventilation tubing system as a second function of at least one of thepressure measurement and the flow measurement in the ventilation system.

The disclosure further describes a pressure support system, such as arespiratory ventilator. The system includes: a pressure generatingsystem adapted to generate a flow of breathing gas; a ventilation tubingsystem including a patient interface device for connecting the pressuregenerating system to a patient; one or more sensors operatively coupledto the pressure generating system or the ventilation tubing system, inwhich each sensor is capable of generating an output indicative of apressure or flow of the breathing gas in the ventilation tubing system;a leak estimation module that identifies leakage in the ventilationtubing system; a delivered lung volume module that calculates aleak-compensated delivered lung volume for a first breath based on theleakage during the first breath and the flow of the breathing gas in theventilation tubing system; a respiratory mechanics calculation modulethat generates a leak-compensated lung compliance based on theleak-compensated delivered lung volume and at least one outputindicative of a pressure of the breathing gas during the first breath;and a pressure control module that causes the pressure generating systemto deliver a second breath to the patient at a target pressurecalculated based on the leak-compensated lung compliance and theleak-compensated delivered lung volume.

The disclosure also describes a controller for a medical ventilator thatincludes a microprocessor, a module (which may be a software programexecuted by the microprocessor, or a component comprising software,hardware and/or firmware that is separate from the microprocessor) thatcalculates leak-compensated delivered lung volume and leak-compensatedlung compliance based on instantaneous elastic leakage and instantaneousinelastic leakage of breathing gas from a ventilation system, and apressure control module that provides pressure regulated volume controlventilation at a pressure determined based on the leak-compensateddelivered lung volume and the leak-compensated lung compliance.

These and various other features as well as advantages whichcharacterize the systems and methods described herein will be apparentfrom a reading of the following detailed description and a review of theassociated drawings. Additional features are set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the technology. Thebenefits and features of the technology will be realized and attained bythe structure particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawing figures, which form a part of this application,are illustrative of described technology and are not meant to limit thescope of the invention as claimed in any manner, which scope shall bebased on the claims appended hereto.

FIG. 1 illustrates an embodiment of a ventilator connected to a humanpatient.

FIG. 2 schematically depicts example systems and methods of ventilatorcontrol.

FIG. 3 illustrates an embodiment of a method of compensating for leakagein a ventilator providing pressure-regulated volume control ventilationto a patient.

FIG. 4 illustrates an embodiment of a method for calculatedleak-compensated lung flow and delivered lung volume while providingpressure-regulated volume control ventilation to a patient.

FIG. 5 illustrates a functional block diagram of modules and othercomponents that may be used in an embodiment of ventilator thatcompensates for elastic and rigid orifice sources of leaks whendetermining the target pressure during pressure-regulated volume controlventilation.

DETAILED DESCRIPTION

Although the techniques introduced above and discussed in detail belowmay be implemented for a variety of medical devices, the presentdisclosure will discuss the implementation of these techniques in thecontext of a medical ventilator providing pressure regulated volumecontrol (PRVC) ventilation to a human patient. The reader willunderstand that the technology described in the context of a medicalventilator for human patients could be adapted for use with othersystems such as ventilators for non-human patients and general gastransport systems in which leaks may cause a degradation of performance.

In PRVC ventilation, a ventilator may evaluate the volume delivered tothe patient over a breath or a specified time period and compare itagainst the volume set by the therapist. If the delivered volume is lessthan the setting, then the pressure target is increased proportionally.

FIG. 1 illustrates an embodiment of a ventilator 20 connected to a humanpatient 24 that is adapted to provide PRVC ventilation. Ventilator 20includes a pneumatic system 22 (also referred to as a pressuregenerating system 22) for circulating breathing gases to and frompatient 24 via the ventilation tubing system 26, which couples thepatient to the pneumatic system via physical patient interface 28 andventilator circuit 30. Ventilator circuit 30 could be a dual-limb orsingle-limb circuit for carrying gas to and from the patient. In adual-limb embodiment as shown, a wye fitting 36 may be provided as shownto couple the patient interface 28 to the inspiratory limb 32 and theexpiratory limb 34 of the circuit 30.

The present systems and methods have proved particularly advantageous innoninvasive settings, such as with facial breathing masks, as thosesettings typically are more susceptible to leaks. However, leaks dooccur in a variety of settings, and the present description contemplatesthat the patient interface may be invasive or non-invasive, and of anyconfiguration suitable for communicating a flow of breathing gas fromthe patient circuit to an airway of the patient. Examples of suitablepatient interface devices include a nasal mask, nasal/oral mask (whichis shown in FIG. 1), nasal prong, full-face mask, tracheal tube,endotracheal tube, nasal pillow, etc.

Pneumatic system 22 may be configured in a variety of ways. In thepresent example, system 22 includes an expiratory module 40 coupled withan expiratory limb 34 and an inspiratory module 42 coupled with aninspiratory limb 32. Compressor 44 or another source(s) of pressurizedgas (e.g., air and oxygen) is coupled with inspiratory module 42 toprovide a gas source for ventilatory support via inspiratory limb 32.

The pneumatic system may include a variety of other components,including sources for pressurized air and/or oxygen, mixing modules,valves, sensors, tubing, accumulators, filters, etc. Controller 50 isoperatively coupled with pneumatic system 22, signal measurement andacquisition systems, and an operator interface 52 may be provided toenable an operator to interact with the ventilator (e.g., changeventilator settings, select operational modes, view monitoredparameters, etc.). Controller 50 may include memory 54, one or moreprocessors 56, storage 58, and/or other components of the type commonlyfound in command and control computing devices.

The memory 54 is computer-readable storage media that stores softwarethat is executed by the processor 56 and which controls the operation ofthe ventilator 20. In an embodiment, the memory 54 comprises one or moresolid-state storage devices such as flash memory chips. In analternative embodiment, the memory 54 may be mass storage connected tothe processor 56 through a mass storage controller (not shown) and acommunications bus (not shown). Although the description ofcomputer-readable media contained herein refers to a solid-statestorage, it should be appreciated by those skilled in the art thatcomputer-readable storage media can be any available media that can beaccessed by the processor 56. Computer-readable storage media includesvolatile and non-volatile, removable and non-removable media implementedin any method or technology for storage of information such ascomputer-readable instructions, data structures, program modules orother data. Computer-readable storage media includes, but is not limitedto, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memorytechnology, CD-ROM, DVD, or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by the computer.

As described in more detail below, controller 50 issues commands topneumatic system 22 in order to control the breathing assistanceprovided to the patient by the ventilator. The specific commands may bebased on inputs received from an operator, the patient 24, the pneumaticsystem 22 and sensors, the operator interface 52 and/or other componentsof the ventilator. In the depicted example, operator interface includesa display 59 that is touch-sensitive, enabling the display to serve bothas an input and output device.

FIG. 2 schematically depicts exemplary systems and methods of ventilatorcontrol. As shown, controller 50 issues control commands 60 to drivepneumatic system 22 and thereby circulate breathing gas to and frompatient 24. The depicted schematic interaction between pneumatic system22 and patient 24 may be viewed in terms of pressure and/or flow“signals.” For example, signal 62 may be an increased pressure which isapplied to the patient via inspiratory limb 32. Control commands 60 arebased upon inputs received at controller 50 which may include, amongother things, inputs from operator interface 52, and feedback frompneumatic system 22 (e.g., from pressure/flow sensors) and/or sensedfrom patient 24.

In an embodiment, before the respiratory mechanics of a patient can bedetermined, the mechanics of the ventilation tubing system may bedetermined. For example, when modeling the delivery of gas to and from apatient 24 via a closed-circuit ventilator, one simple assumption isthat compliance of the ventilator circuit 30 (the “circuit compliance”)is fixed and that all gas injected into the ventilator circuit 30 thatdoes not exit the circuit 30 via the expiratory limb 34 (in a dual-limbembodiment) fills the circuit as well as the patient's lungs and causesan increase in pressure. As gas is injected (L₁), the lung responds tothe increased gas pressure in the circuit 30 by expanding. The amountthe lung expands is proportional to the lung compliance and is definedas a function of gas pressure differential (e.g., lung compliance=volumedelivered/pressure difference). As discussed in greater detail below,this assumption is not valid when leaks are present.

The term circuit compliance is used to refer to the relationship betweenthe pressure in the ventilator circuit 30 (or ventilator circuit 30 andattached patient interface 28, depending on how the compliance isdetermined) changes based on changes in volume delivered into thecircuit. In an embodiment, the circuit compliance may be estimated bypressurizing the ventilator circuit 30 (or circuit 30 and interface 28combination) when flow to the patient is blocked and measuring thevolume of additional gas introduced to cause the pressure change(compliance=volume delivered/pressure difference).

The term circuit resistance is used to refer to the amount the pressurechanges between two sites upstream and downstream the ventilator circuitas a function of volumetric flow rate through that circuit. Circuitresistance may be modeled as a two-parameter function of flow andseveral methods for modeling and calculating circuit resistance areknown in the art. For example, in an embodiment, the circuit resistancemay be estimated by passing several fixed flow rates through the circuitand measuring the pressure difference between certain upstream anddownstream sites and finding the best curve fit to the collected data.

Methods of determining circuit compliance and circuit resistance (suchas those described above) may be executed by the operator prior toattaching the patient to the ventilator as part of the set up of theventilator 20 to provide therapy. Other methods of determining circuitcompliance and/or resistance during therapy are also known and could beadapted for use with the disclosed leak-compensation systems and methodsdescribed herein.

In many cases, it may be desirable to establish a baseline pressureand/or flow trajectory for a given respiratory therapy session. Thevolume of breathing gas delivered to the patient's lung (L₁) and thevolume of the gas exhaled by the patient (L₂) are measured ordetermined, and the measured or predicted/estimated leaks are accountedfor to ensure accurate delivery and data reporting and monitoring.Accordingly, the more accurate the leak estimation, the better thebaseline calculation of delivered and exhaled flow rates and volumes.

Errors may be introduced due to leaks in the ventilation tubing system26. The term ventilation tubing system 26 is used herein to describe theventilator circuit 30, any equipment attached to or used in theventilator circuit 30 such as water traps, monitors, drug deliverydevices, etc. (not shown), and the patient interface 28. Depending onthe embodiment, this may include some equipment contained in theinspiration module 42 and/or the expiration module 40. When referring toleaks in or from the ventilation tubing system 26, such leaks includeleaks within the tubing system 26 and leaks where the tubing system 26connects to the pressure generator 22 or the patient 24. Thus, leaksfrom the ventilation tubing system 26 include leaks from the ventilatorcircuit 30, leaks from the patient interface 28 (e.g., masks are oftenprovided with holes or other pressure relief devices through which someleakage may occur), leaks from the point of connection of the patientinterface 28 to the patient 24 (e.g., leaks around the edges of a maskdue to a poor fit or patient movement), and leaks from the point ofconnection of the patient interface 28 to the circuit 30 (e.g., due to apoor connection between the patient interface 28 and the circuit 30).

For the purpose of estimating how a leak flow rate changes based onchanges in pressure in the ventilation tubing system 26, theinstantaneous leak may be modeled as a leak through a single rigidorifice or opening of a fixed size in which that size is determinedbased on comparing the total flow into the inspiratory limb 32 and outof the expiratory limb 34. However, this leak model does not take intoaccount any elastic component of leak source(s) in the system 26, thatis how much of the area of any of the holes or openings in theventilation tubing system 26 through which leakage occurs may change dueto an increase or decrease in pressure.

It has been determined that not accounting for elastic leakage from theventilation tubing system 26 can cause many problems. First, if only theinelastic/fixed orifice model is used to estimate leak, the subsequenterrors caused by ignoring the elastic effects of any actual leaks end upgenerating inaccurate estimates of flow rates into the lung. This cancause the ventilator 20 to estimate gas volume delivered into the lunginaccurately when, in fact, the elastic leaks in the system 26 have letmore gas escape than estimated. Second, if the elasticity of the leaksource is ignored, any other calculation, estimate, or action that theventilator 20 may perform which is affected by the leak estimate will beless accurate.

In the systems and methods described herein, the provision of PRVCventilation is made more accurate by compensating for leakage from theventilation tubing system. In the embodiments described herein fixed(rigid) and elastic components of the system leakage are used whendetermining the lung flow, net lung volume, lung compliance and lungresistance of the patient. This results in a more accurate determinationof lung compliance and lung resistance and, therefore, ventilation ofpatients based on respiratory mechanics. While the systems and methodsare presented in the context of specific leakage models, the technologydescribed herein could be used to compensate the respiratory mechanicsdetermined by any model for leakage using any type of mechanicalventilator or other device that provides gas.

FIG. 3 illustrates an embodiment of a method of compensating PRVCventilation for leakage during delivery of gas from a medical ventilatorto a patient. In the method 300 shown, a medical ventilator such as thatdescribed above with reference to FIGS. 1 and 2 is used to provide PRVCventilation to a patient.

The method 300 illustrated starts with a setup operation 301 in whichthe operator directs the ventilator to provide PRVC ventilation. In thesetup operation 301, the operator selects a volume of gas to bedelivered to the patient, that is to be delivered into the lung, over aspecified time period such as a minute, a number of breaths, etc. Thisdesired delivered volume is received by the ventilator and stored inmemory for use during PRVC ventilation.

In the embodiment shown, the method 300 includes a circuit complianceand resistance operation 302. In that operation 302, the ventilatorcircuit compliance and resistance are estimated. In an embodiment, thismay be performed prior to connecting the ventilator to the patient (aspreviously described). Alternatively, it may be dynamically determinedperiodically throughout the delivery of ventilatory support to thepatient. The circuit compliance and resistance may be used in theremaining operations to correct for any losses in volume or effects onthe volume delivered to the patient introduced due to the patientcircuit.

After the circuit compliance and resistance have been determined, theventilator is connected to the patient and an initialization operation304 is performed. In the initialization operation 304 the ventilatoroperates for an initialization period in order to generate an initialestimate of lung compliance. If the ventilator already has someknowledge of the respiratory mechanics of the patient (e.g., therespiratory mechanics have been recently determined during provision ofa different type of ventilation or an operator has provided initialsettings for lung compliance and resistance), this operation 304 may beautomatically or manually omitted in favor of the previously determinedvalues.

A description of an embodiment of the initialization operation 304 is asfollows. Because the ventilator does not know the patient's mechanicswhen the PRVC breath type is selected, it performs a startup routine toobtain initial data. In an embodiment, upon startup the ventilatordelivers some number (e.g., two, four, etc.) of consecutivepressure-based breaths. One or more of these initial breaths given inthe startup period may also include an end-inspiratory or other maneuverthat yields estimates of the patient's lung compliance. Using fourtraining breaths for the initialization operation 304 as an example, thefirst breath is delivered using a predicted resistance for theartificial airway and conservative estimates for patient lungcompliance. The predicted values may be determined based on knowncharacteristics of the patient, such as based on the patient's idealbody weight (IBW), height, gender, age, physical condition, etc. Each ofthe following three pressure-based breaths averages stepwise decreasedphysiologic values with the estimated lung compliance values from theprevious breaths, weighting earlier estimates less with each successivebreath, and yielding more reliable estimates for lung compliance. Othermethods may be used to find a first estimate of the lung compliance andupdate it on an ongoing basis as ventilation continues.

In an embodiment of the method 300, a leakage estimate may also be doneprior to the initialization operation 304. Prior determination of leakparameters allows for estimates of respiratory mechanics to be made.This may include delivering pressure-regulated breaths with specificsettings or performing specific “leak maneuvers”, that is a specifiedset of controlled actions on the part of the ventilator that allowleakage parameters to be identified and quantified, such as interruptingthe therapeutic delivery of respiratory gas and holding or changing thepressure and flow, so that data concerning the leakage of the systemduring the controlled actions may be obtained. For example, a leakmaneuver may include periodically holding the pressure and flow in thecircuit constant while determining (based on a comparison of themeasured flow into the inspiratory limb and the measured flow out of theexpiratory limb via the exhalation valve) the net leakage from thesystem. In an embodiment, such a leak maneuver may be performed duringspecific parts of the respiratory phase, e.g., at the end of theexpiratory phase. In yet another embodiment, a sequence ofpressure-based test breaths is delivered with specific settings todetermine leak parameters prior to execution of test breaths forrespiratory mechanics determinations.

After the initialization operation 304, the ventilator provides ongoingPRVC ventilation to the patient in a PRVC ventilation operation 306. Asdiscussed above, during PRVC ventilation the ventilator calculates atarget pressure to be delivered to the patient during inspiration basedon the desired lung volume to be delivered and the leak-compensated lungcompliance of the patient. When in an inspiratory phase, the ventilatorraises the pressure in the ventilator tubing system so that the targetinspiratory pressure is applied. During exhalation, the pressure may bedropped to some pre-selected positive end expiratory pressure (PEEP)level or to atmospheric level depending on the desires of the operator.The duration of the inspiratory and expiratory phases may be determinedbased on patient effort or based on a preselected inspiratory time.

As described above, the target pressure delivered during eachinspiratory phase is determined based on the desired lung volume to bedelivered and leak-compensated lung compliance of the patient. In orderto compensate for leakage in the circuit, in the method 300 shown thePRVC ventilation operation 306 includes the ongoing calculation ofleakage while providing ventilation, as illustrated by the leakagecalculation/compensation operation 307. As discussed in greater detailbelow with reference to FIG. 4, the leakage is calculated and theleak-compensated values for lung flow and current lung volume (i.e., thevolume of gas in the lung at that moment) are determined taking intoaccount the calculated leakage.

The method 300 also includes determining the leak-compensated deliveredlung volume in a delivered volume calculation operation 308. Thedelivered volume calculation operation 308 uses the leak-compensatedlung flow and net lung volume to determine how much gas was delivered tothe lungs of the patient during the breath. In an embodiment, thisoperation 308 is an ongoing operation in which the volume of gasdelivered to the patient is accumulated over the course of theinspiratory phase, so that upon completion of the inspiratory phase theaccumulated volume is the total delivered volume for the breath.Alternatively, this operation 308 may be performed as soon as theinspiratory phase ends, any time during the expiratory phase or at thebeginning of the next inspiratory phase using leak-compensated lung flowor volume data collected during the breath.

The leak-compensated delivered lung volume for the breath is then usedin a lung compliance calculation operation 310 to calculate aleak-compensated lung compliance for the patient. In an embodiment, thelung compliance calculation operation 310 includes using the followingequation to determine lung compliance:Leak-Compensated Lung Compliance=V/(EIP−EEP)in which V is the leak-compensated delivered lung volume for the breath,EIP is the pressure at the end of the inspiratory phase of the breath,and EEP is the pressure at the end of the expiratory phase of thebreath. As mentioned above, these pressure values are determined fromthe monitoring of pressure and/or flow during the PRVC ventilationoperation 306.

The above equation is but one example of a method of determined lungcompliance from parameters monitored by a ventilator such as pressure,flow and volume. Any suitable method may be used as long as themonitored parameters are compensated for the leakage identified in theleakage calculation/compensation operation 307 so that aleak-compensated lung compliance is obtained.

In an embodiment, the delivered volume calculation operation 308 andlung compliance calculation operation 310 may be performed as a singleoperation at the same time instead of separately as shown.

Based on the leak-compensated lung compliance, the system thencalculates a target pressure for the next breath in a calculate targetpressure operation 312. This calculation takes into account theleak-compensated volume of gas delivered to the patient's lungs duringthe last breath to determine if sufficient volume is being deliveredrelative to the desired volume identified by the operator. Thiscalculation also may take into account the leak-compensated volume ofgas delivered during earlier breaths depending on the time period overwhich the desired volume is to be delivered. Depending on the comparisonof the leak-compensated delivered lung volume and the desired deliveredlung volume, the target pressure to be used for the next breath may beraised or lowered relative to the current pressure. The amount thetarget pressure is raised or lowered is a function of theleak-compensated lung compliance, the predicted or known inspiratoryphase duration and ventilator settings as well as patient safetyprecautions.

The newly determined values of lung compliance and lung resistance maybe averaged, low-pass filtered or otherwise combined with the previouslydetermined values. These revised values are then stored for use in laterdelivery of PRVC ventilation.

In an embodiment, the delivered volume calculation operation 308, lungcompliance calculation operation 310 and calculate target pressureoperation 312 may be performed as a single operation at the same timeinstead of separately as shown.

After calculating the revised target pressure, upon the next inspirationthe ventilator then provides the revised target pressure to the patient,illustrated in FIG. 3 by the flow returning to the PRVC ventilationoperation 306. The method 300 is then repeated from that point until thedelivery of PRVC ventilation is terminated by some outside action, suchas, change of mode setting or failure to converge on an acceptable lungcompliance value.

FIG. 4 illustrates an embodiment of a method for calculatingleak-compensated parameters while providing PRVC ventilation to apatient. In an embodiment, the method 400 corresponds to the operationsperformed during the leakage determination operation 306 discussed withreference to FIG. 3. In the embodiment of the method 400 illustrated,the operations occur repeatedly while the ventilator is providing PRVCventilation, such as once a sample period or computation cycle, whilethe ventilator is providing either the target pressure (during theinspiratory phase) or an expiratory pressure such as PEEP (during theexpiratory phase).

During PRVC ventilation, the pressure and flow and other parameters ofthe system are monitored, illustrated by the monitoring operation 402.In an embodiment, the monitoring operation 402 collects data includingthe instantaneous pressure and/or flow at or indicative of one or morelocations in the ventilation tubing system. Depending upon how aparticular leak model is defined, the operation 402 may also includemaking one or more calculations using data from pressure and flowmeasurements taken by the sensors. For example, a model may require aflow measurement as observed at the patient interface even though theventilation system may not have a flow sensor at that location in theventilation tubing system. Thus, a measurement from a sensor or sensorslocated elsewhere in the system (or data from a different type of sensorat the location) may be mathematically manipulated in order to obtain anestimate of the flow observed at the patient interface in order tocalculate the leak using the model.

The data obtained in the monitoring operation 402 is then used tocalculate leakage from the ventilator tubing system in a leakagecalculation operation 404. In an embodiment, the leakage calculationoperation 404 uses the data obtained in the monitoring operation 402,e.g., some or all of the instantaneous pressure and flow data collectedduring the monitoring operation 402 as well as information about thecurrent respiratory phase (inhalation or exhalation).

The leakage calculation operation 404 calculates an instantaneousleakage flow or volume for the sample period. The instantaneous leakageis calculated using a mathematical formula that has been previouslydetermined. In an embodiment, the mathematical formula is a leakagemodel that separates the leak into the sum of two leak components,inelastic leak and elastic leak, in which each component represents adifferent relationship between the quantity of leakage from theventilation system and the measured current/instantaneous pressureand/or flow of gas in the ventilation system. As discussed above, theinelastic leak may be modeled as the flow through a rigid orifice of afixed size while the elastic leak may be modeled as the flow through adifferent orifice of a size that changes based on the pressure (or flow)of the gas in the ventilation system.

An example of a method and system for modeling leak in a ventilationsystem as a combination of an elastic leak component and an inelasticleak component can be found in commonly-assigned U.S. Provisional PatentApplication Ser. No. 61/041,070, filed Mar. 31, 2008, titled VENTILATORLEAK COMPENSATION, which application is hereby incorporated by referenceherein. The VENTILATOR LEAK COMPENSATION represents one way ofcharacterizing the leak from a ventilation system as a combination ofelastic and inelastic components. Other methods and models are alsopossible and may be adapted for use with this technology.

The mathematical formula used to calculate leakage may contain severalparameters that are empirically determined and that may be periodicallyor occasionally revised in order to maintain the accuracy of the leakageestimate. For example, in an embodiment the parameters of a leakageformula include a first constant associated with the rigid orifice and asecond constant associated with the variable-sized orifice. At varioustimes during ventilation, the calculated leakage may be checked againsta measured leakage and, if the estimate is significantly different fromthe measured leakage, the constants may be revised. This revision of theparameters in a leakage formula may be done as part of the leakagecalculation operation 404 or may be done as a separate operation (notshown) that may, or may not, be performed every sample period.

The term instantaneous is used herein to describe a determination madefor any particular instant or sampling period based on the measured datafor that instant. For example, if a pressure measurement is taken every5 milliseconds (sample period), the pressure measurement and the leakmodel can be used to determine an instantaneous leak flow based on theinstantaneous pressure measurement. With knowledge of the length of thesample period, the instantaneous flow may then be used to determine aninstantaneous volume of gas leaking out of the circuit during thatsample period. For longer periods covering multiple sample periods theinstantaneous values for each sample period may be summed to obtain atotal leakage volume. If a measurement is also the most recentmeasurement taken, then the instantaneous value may also be referred toas the current value.

After the current leak has been calculated, the method 400 furtherestimates the leak-compensated instantaneous lung flow to or from thepatient in a lung flow estimation operation 406. The estimated lung flowis compensated for the leak flow calculated in the instantaneous leakcalculation operation 404 so that it represents a more accurate estimateof the actual flow into (or out of depending on the point of view andperiod selected) the lungs of the patient.

In the embodiment illustrated, the leak-compensated net and deliveredlung volumes are also calculated as part of the lung flow estimationoperation 406. In an embodiment, this may be performed by maintaining arunning summation of net flow into/out of the lung over the period of abreath and a running summation of the flow delivered to the patientduring the inspiratory phase. For example, upon triggering inhalation,the ventilator may set a variable corresponding to net lung volume tozero and, each sample period, update this net lung volume to include thedetected leak-compensated instantaneous lung flow delivered to thepatient during that sample period. Likewise, the ventilator may also seta variable corresponding to delivered lung volume to zero and, eachsample period during the inspiratory phase, update this net lung volumeto include the detected leak-compensated instantaneous lung flow intothe patent, if any, during that sample period.

In the PRVC ventilation method 400 illustrated, the leak-compensatedlung flow or delivered lung volume will ultimately be used to calculatea leak-compensated lung compliance as described in FIG. 3. Ultimately,this leak-compensated lung compliance along with other leak-compensateddata will be used to determine the target pressure for the nextinspiratory phase.

The method 400 is then repeated every computational cycle or sampleperiod, as illustrated by the feedback loop, so that theleak-compensated instantaneous lung flow and leak-compensated deliveredlung flow are continuously determined during PRVC ventilation.

The following is a discussion of two embodiments of methods forcompensating the estimation of respiratory mechanics for leaks. Thefirst embodiment is that of applying leak compensation to a staticcompliance and resistance determination. The second embodiment is thatof applying leak compensation to a dynamic compliance determination.

FIG. 5 illustrates a functional block diagram of modules and othercomponents that may be used in an embodiment of ventilator thatcompensates for elastic and rigid orifice sources of leaks whendetermining the target pressure during PRVC ventilation. In theembodiment shown, the ventilator 500 includes pressure sensors 506 (twoare shown placed at different locations in the system), flow sensors(one is shown), and a ventilator control system 502. The ventilatorcontrol system 502 controls the operation of the ventilator and includesa plurality of modules described by their function. In the embodimentshown, the ventilator control system 502 includes a processor 508,memory 514 which may include mass storage as described above, a leakestimation module 512 incorporating a parametric leak model accountingfor both elastic and rigid orifice leak sources such as that describedin U.S. Provisional Application 61/041,070 previously incorporatedherein, a target pressure calculation module 516, a pressure and flowcontrol module 518, a monitoring module 522, a leak-compensated lungcompliance module 524, and a leak-compensated lung flow and volumeestimation module 526. The processor 508 and memory 514 have beendiscussed above. Each of the other modules will be discussed in turnbelow.

The main functions of the ventilator such as receiving and interpretingoperator inputs and providing therapy via changing pressure and flow ofgas in the ventilator circuit are performed by the control module 518.In the context of the methods and systems described herein, the module518 will perform one or more actions upon the determination that apatient receiving therapy is inhaling or exhaling.

In the embodiment described herein, the control module 518 determinesand provides the appropriate pressure to the patient when in PRVCventilation mode. This may include performing one or more calculationsbased on leak-compensated lung flow, leak-compensated lung volume,leak-compensated lung compliance and leak-compensated lung resistance.

The calculation of the target pressure to provide during the inspiratoryphase of a breath is performed by the target pressure calculation module516. The target pressure is calculated based on the therapist-selecteddesired lung volume and the leak-compensated delivered lung volume. Themodule 516 utilizes one or more respiratory models suitable fordetermination of target pressure based on monitored parameters and/orleak-compensated respiratory mechanics such as lung compliance. Themodule 516 uses leak-compensated values for one or both of lung flow anddelivered lung volume when calculating the target pressure, depending onthe method used by the module. Leak-compensated values may be retrievedif they have already been calculated or may be calculated as needed fromleakage information received from the leak-compensated lung flow and netlung volume estimation module 526.

The dynamic calculation of lung compliance is performed by theleak-compensated lung compliance calculation module 524. The module 524utilizes one or more dynamic respiratory models, such as that describedabove with reference to lung compliance calculation operation 310 ofFIG. 3, to calculate leak-compensated lung compliance. The module 524uses leak-compensated values for one or both of lung flows and deliveredlung volume when calculating lung compliance. Leak-compensated valuesmay be retrieved if they have already been calculated or may becalculated from leakage information received from the leak-compensatedlung flow and delivered lung volume estimation module 526.

The current conditions in the ventilation system are monitored by themonitoring module 522. This module 522 collects the data generated bythe sensors 504, 506 and may also perform certain calculations on thedata to make the data more readily usable by other modules or mayprocess the current data and or previously acquired data or operatorinput to derive auxiliary parameters or attributes of interest. In anembodiment, the monitoring module 522 receives data and provides it toeach of the other modules in the ventilator control system 502 that needthe current pressure or flow data for the system.

In the embodiment shown, leak-compensated lung flow and delivered lungvolume are calculated by the lung flow module 526. The lung flow module526 uses a quantitative model for lung flow of the patient during bothinhalation and exhalation and from this characterization and pressureand flow measurements generates an estimate for instantaneous lung flow.In an embodiment, lung flow may be simply determined based onsubtracting the estimated leak flow and measured outflow via theexpiratory limb from the flow into the inspiratory limb, therebygenerating a leak-compensated net flow into (or out of) the lung. Thelung flow module 526 may or may not also calculate a leak-compensateddelivered lung volume for a patient's breath as described above.Compression in the circuits and accessories may also be accounted for toimprove the accuracy of estimated lung flow.

The leak model parameters are generated by the leak estimation module512 which creates one or more quantitative mathematical models,equations or correlations that uses pressure and flow observed in theventilation system over regular periods of respiratory cycles(inhalation and exhalation) and apply physical and mathematicalprinciples derived from mass balance and characteristic waveformsettings of ventilation modalities (regulated pressure or flowtrajectories) to derive the parameters of the leak model incorporatingboth rigid and elastic (variable pressure-dependent) orifices. In anembodiment, the mathematical model may be a model such as:Q _(inelastic) =R ₁ *P _(i) ^(x)Q _(elastic) =R ₂ *P _(i) ^(y)wherein Q_(elastic) is the instantaneous leak flow due to elastic leaksin the ventilation system, Q_(inelastic) is the instantaneous leak flowdue to inelastic leaks in the ventilation system, R₁ is the inelasticleak constant, R₂ is the elastic leak constant, P_(i) is the current orinstantaneous pressure measurement, x is an exponent for use whendetermining the inelastic leak and y is an exponent different than x foruse when determining the elastic leak. The group R₁*P_(i) ^(x)represents flow through an orifice of fixed size as a function ofinstantaneous pressure P_(i) and the group R₂*P_(i) ^(y) represents flowthrough a different orifice that varies in size based on theinstantaneous pressure. The equations above presuppose that there willalways be an elastic component and an inelastic component of leakagefrom the ventilation system. In the absence of an elastic component or aleak source of varying size, R₂ would turn out be zero.

In the embodiment shown, the current or instantaneous elastic leak iscalculated by the leak estimation module 512. The calculation is madeusing the elastic leak portion of the leak model developed by the leakestimation module 512 and the pressure data obtained by the monitoringmodule 522. The leak estimation module 512 may calculate a newinstantaneous elastic leak flow or volume for each pressure sample taken(i.e., for each sampling period) by the monitoring module 522. Thecalculated elastic leak may then be provided to any other module asneeded.

In the embodiment shown, the current or instantaneous inelastic leak isalso calculated by the leak estimation module 512. The calculation ismade using the inelastic leak portion of the leak model and the pressuredata obtained by the monitoring module 522. The leak estimation module512 may calculate a new instantaneous inelastic leak flow or volume foreach pressure sample taken (i.e., for each sampling period) by themonitoring module 522. The calculated inelastic leak may then beprovided to any other module as needed.

The system 500 illustrated will compensate lung flow for leaks due toelastic and inelastic leaks in the ventilation system. Furthermore, thesystem may perform a dynamic compensation of lung flow based on thechanging leak conditions of the ventilation system and the instantaneouspressure and flow measurements. The system then compensates the lungcompliance and target pressure calculations based on the estimatedleakage in the system. By compensating for the inelastic as well as theelastic components of dynamic leaks, the medical ventilator can moreaccurately and precisely a target pressure so that the desired lungvolume selected by the therapist is achieved.

Furthermore, embodiments of the systems and methods described above mayalso include checks and balances based on patient type and knowncharacteristics (e.g., Ideal Body Weight, etc.). For example, acalculated pressure target (or change between the current and the newlycalculated pressure target to be used in the next inspiration) may becompared against a safety criteria based on Ideal Body Weight, age,gender, patient parameters determined during ventilation or operatorprovided safety thresholds.

If the comparison indicates the newly calculated pressure is unsafe, theventilator may perform one or more safety actions. In an embodiment,such safety actions may include limiting stepwise changes in desiredpressure target and generating alarms or warnings. Delivery of PRVC mayalso be terminated in situations deemed unsafe for the patient or whenacceptable data are not available (e.g, when the process of lungcompliance estimation fails to converge to an acceptable value). In sucha situation the ventilator may switch to some other mode, such as apressure support mode or volume control mode, than PRVC. The modeswitched may be determined by the operator when setting up the PRVCventilation or may be a default mode selected by the manufacturer.

It will be clear that the systems and methods described herein are welladapted to attain the ends and advantages mentioned as well as thoseinherent therein. Those skilled in the art will recognize that themethods and systems within this specification may be implemented in manymanners and as such is not to be limited by the foregoing exemplifiedembodiments and examples. For example, the operations and steps of theembodiments of methods described herein may be combined or the sequenceof the operations may be changed while still achieving the goals of thetechnology. In addition, specific functions and/or actions may also beallocated in such as a way as to be performed by a different module ormethod step without deviating from the overall disclosure. In otherwords, functional elements being performed by a single or multiplecomponents, in various combinations of hardware and software, andindividual functions can be distributed among software applications. Inthis regard, any number of the features of the different embodimentsdescribed herein may be combined into one single embodiment andalternate embodiments having fewer than or more than all of the featuresherein described are possible.

While various embodiments have been described for purposes of thisdisclosure, various changes and modifications may be made which are wellwithin the scope of the technology described herein. For example, thesystems and methods described herein could be adapted to periodicallyperform a static respiratory mechanics maneuver to obtain a moreaccurate lung compliance estimate in order to check the dynamicallydetermined leak-compensated lung compliance. Numerous other changes maybe made which will readily suggest themselves to those skilled in theart and which are encompassed in the spirit of the disclosure and asdefined in the appended claims.

What is claimed is:
 1. A pressure support system comprising: a pressuregenerating system adapted to generate a flow of breathing gas; aventilation tubing system including a patient interface device forconnecting the pressure generating system to a patient; one or moresensors operatively coupled to the pressure generating system or theventilation tubing system, each sensor capable of generating an outputindicative of a pressure or flow of the breathing gas in the ventilationtubing system; a leak estimation module that identifies an elasticleakage in the ventilation tubing system as a first function of anoutput of at least one sensor and an inelastic leakage in theventilation tubing system as a second function of an output of at leastone sensor; a delivered lung volume module that calculates aleak-compensated delivered lung volume for a first breath based on theleakage during the first breath and the flow of the breathing gas in theventilation tubing system; a respiratory mechanics calculation modulethat generates a leak-compensated lung compliance based on theleak-compensated delivered lung volume and at least one outputindicative of a pressure of the breathing gas during the first breath;and a pressure control module that causes the pressure generating systemto deliver a second breath to the patient at a target pressurecalculated based on the leak-compensated lung compliance and theleak-compensated delivered lung volume.
 2. The system of claim 1 furthercomprising: a memory storing a desired delivered lung volume provided bya user.
 3. The system of claim 2 wherein the pressure regulated volumecontrol module calculates the target pressure based on the desireddelivered lung volume, the leak-compensated lung compliance and theleak-compensated delivered lung volume.
 4. The system of claim 1 whereinthe respiratory mechanics calculation module generates theleak-compensated lung compliance based on the leak-compensated deliveredlung volume and a pressure difference.
 5. The system of claim 4 whereinthe pressure difference is a difference between an end inspiratorypressure of the first breath and an end expiratory pressure of the firstbreath.
 6. The system of claim 1 wherein the delivered lung volumemodule calculates the leak-compensated delivered lung volume for a firstbreath based on the elastic and inelastic leakage during the firstbreath and the flow of the breathing gas in the ventilation tubingsystem.
 7. A controller for a medical ventilator comprising: amicroprocessor; a leak estimation module that identifies aninstantaneous inelastic leakage in a ventilation tubing system as afirst function of an output of at least one sensor; and an instantaneouselastic leakage in the ventilation tubing system as a second function ofan output of at least one sensor; a module that calculatesleak-compensated delivered lung volume and leak-compensated lungcompliance based on the instantaneous elastic leakage and theinstantaneous inelastic leakage of breathing gas from the ventilationtubing system; and a pressure control module that provides pressureregulated volume control ventilation at a pressure determined based onthe leak-compensated delivered lung volume and the leak-compensated lungcompliance.
 8. The controller of claim 7 further comprising: a memorythat stores an identification of a target volume of gas to be deliveredduring pressure regulated volume control ventilation; and wherein thepressure control module provides pressure regulated volume controlventilation at a pressure determined based on the leak-compensateddelivered lung volume, the leak-compensated lung compliance, and thetarget volume of gas.